Kaipara is 'last harbour standing'
Young west coast snapper "hot spots" in the Kaipara Harbour are being investigated as concerns about their feeding grounds continue.
"The Kaipara is the major harbour for west coast juvenile snapper habitats, so it is a really key place," NIWA Auckland marine ecologist Mark Morrison says.
"We know it's under stress from sedimentation and other land-based impacts that degrade key habitats."
These habitats include seagrass meadows and horse mussel beds, which have now largely disappeared from most harbours in northwest New Zealand, Auckland Council marine scientist Jarrod Walker says.
They are thought to have been smothered by silt from roading and building sites, and from farming and forestry. The effects of plankton blooms caused by nutrients washed off the land may also have contributed, he says.
Manukau and other west coast harbours were also once important nurseries for species like the west coast snapper. But their seagrass meadows have virtually gone. Kaipara is the "last harbour standing" as a snapper nursery on the west coast, Dr Walker says.
The harbour still has broad expanses of seagrass meadows which can support high numbers of juvenile snapper, trevally, parore, spotties, piper, pipefish, and other species. These are feeding grounds and also give them protection from predators.
Sediment affects cockles and scallops in the harbour by reducing the time spent eating as they have to sort through the sediment for food - so they're smaller and not so robust. This also affects their reproduction as they need to be a good size to breed.
Much of the sediment comes down the Northern Wairoa River, some of it reaching the southern Kaipara, Dr Walker says.
Potentially this has an impact on the seagrass meadow just south of the Hoteo River, also a sediment source.
The worst culprit in the south is the Kaipara River which has high silt levels for its size. The river drains from the Waitakeres.
Measures are under way to reduce runoff sediment with some farms around the harbour planting and using other land management practises as part of work by the Integrated Kaipara Harbour Management Group. This group includes the Auckland Council, Kaipara and Whangarei District Councils, Northland Regional Council, conservation groups, the Department of Conservation, the Ministry of Primary Industries, NIWA and Land Care Research, iwi, and Fonterra.
NIWA took seagrass meadow samples in the southern harbour during March.
Scientists mapped shallow water habitats using aerial photography, showing seagrass areas.
The study will examine what components of these habitats are most important to small fish, Dr Morrison says.
The maps also show patches of Asian date mussels, an invasive species that has formed large beds.
Combined with fish sampling, they show where fish nurseries are, and allow monitoring of their health through cost-effective remote sensing.
"In the past, we have done broad scale fish studies across many harbours, but this study will provide very detailed information on New Zealand's largest harbour, linking fish to fish habitats identified by remote sensing," Dr Morrison says.
Scientists collected fish using small, hand-hauled, fine-mesh beach seines, pulled up onto a specially designed ramp deployed from an oyster barge.
They sampled 80 to 100 sites across 300 square kilometres in the southern Kaipara.
"The research will then help us identify which factors are most critical, so that the most important habitats can be identified, mapped, and protected," Dr Morrison says.
The aerial photography was funded by the Ministry for Primary Industries because these habitats are important for fish stock and ecosystem based fisheries management and marine spatial planning. The fish sampling research is funded by the Ministry of Business, Innovation and Employment.
Regular testing of suspended sediment in the harbour goes back 20 years.